The present invention relates to process for removing contaminants from gas streams in carbon dioxide production and purification processes.
During the operation of carbon dioxide purification plants, different sources of carbon dioxide are employed. These sources can vary widely in terms of the content of impurities present therein that need to be removed. This is particularly important in carbon dioxide that is to be beverage grade liquid carbon dioxide. Typically these contaminants include various hydrocarbons, non-condensable compounds and sulfur compounds. The carbon dioxide concentration in the feed of the stream to be purified will range between 10 and 99.9 volume %.
Typical technologies used to remove trace amounts of hydrocarbons and sulfur compounds (approximately <200 ppm in the feedgas) are adsorption and distillation. Absorption can also be utilized for water soluble hydrocarbons.
Higher concentrations of insoluble hydrocarbons are typically treated by catalytic oxidation. In these processes, oxygen is added to the feedgas and the carbon dioxide is preheated in a cross heat exchanger before entering a feedgas heater to reach the typical reactor inlet temperature of 450° C. The hydrocarbons are efficiently converted to carbon dioxide and water vapor in a reactor containing a noble metal catalyst. Heat is recovered in the cross heat exchanger and the product is cooled further in an aftercooler which can be a two stage system against cooling water and against refrigerant.
The hydrocarbon conversion is fairly efficient at an excess oxygen concentration at the outlet of greater than 500 ppm. However, methane conversion can be deficient and is usually addressed by a downstream distillation process. Further the cost for a catalytic oxidation unit can be expensive in terms of its installation. Because of the high temperatures and the water vapor present, high grade metallurgy is required in the heat exchangers and reactor vessel. Further the use of a palladium/platinum catalyst adds significant cost to the overall unit.
If the hydrocarbon content in the feedgas is sufficiently high such as greater than 2000 to 3000 ppmv hydrocarbon in the feedgas, the unit can operate autothermally, i.e., the heat recovered in the cross heat exchangers is sufficient to pre-heat the feedgas to the reactor inlet temperature and the feedgas heater is only required for start-up operation.
For lower hydrocarbon contents typically less than 2000 ppmv, the preheater needs to be in operation continuously which will add to the energy consumption for the process.
Historically, catalytic oxidation units were placed in plants which had a proven high hydrocarbon content in the feedgas thereby allowing for autothermal operation of the unit. A more recent development driven by the steadily increasing quality requirements for beverage grade liquid carbon dioxide product is to include catalytic oxidation unit in plants which have lower and/or uncertain hydrocarbon concentrations in the feedgas. This development leads to a safe design for plants to handle hydrocarbons in the feedgas but increases capital and operating costs of the plant significantly.